1. Field of the Invention
The present invention relates to an optical waveguide element and especially relates to an optical waveguide element used preferably for an optical waveguide device of a traveling-wave type which can perform a high speed modulation.
2. Description of Related Art
Recently, the optical waveguide device has been used as an external modulator according to an improvement of optical fiber communication system which permits high speed and large capacity signal transmissions. Since the external modulator mentioned above is used under a condition such that a high speed switching operation is required, the optical waveguide device used as the external modulator needs a high speed modulating property. To this end, there have been known some techniques such that an electrode, for applying the modulation signal, of the optical waveguide element which constructs the optical waveguide device is shaped particularly or that a buffer layer made of for example silicon dioxide is formed between the electrode and a substrate which constructs the optical waveguide element.
However, in the techniques mentioned above, there are drawbacks such that a producing cost is increased since producing steps of the electrode are complicated and that a positioning precision of the electrode is deteriorated when the buffer layer is formed. In order to eliminate the drawbacks mentioned above, the applicant disclosed, in Japanese Patent Laid-Open Publication No. 10-133159 (JP-A-10-133159), an optical waveguide element wherein a thickness of a portion of the substrate, which constructs the optical waveguide element, at least positioned correspondingly to the electrode is made thin so as to perform a high speed modulation.
However, in the case that the optical waveguide element mentioned above is used actually as the optical waveguide device in such a manner that an optical fiber is connected to the optical waveguide element and an electrical connector is further connected to the optical waveguide element so as to apply a modulation signal to the electrode, since a characteristic impedance of the optical waveguide device is relatively smaller than that of the modulation signal, an impedance matching therebetween is not sufficient. Therefore, there is a drawback such that a high frequency loss is increased.